/*
* Copyright (C) 2011 The Guava Authors
*
* Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
* in compliance with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software distributed under the License
* is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
* or implied. See the License for the specific language governing permissions and limitations under
* the License.
*/
package com.google.common.hash;
import java.nio.charset.Charset;
import com.google.common.annotations.Beta;
import com.google.common.primitives.Ints;
/**
* A hash function is a collision-averse pure function that maps an arbitrary
* block of data to a number called a <i>hash code</i>.
*
* <h3>Definition</h3>
*
* <p>
* Unpacking this definition:
*
* <ul>
* <li><b>block of data:</b> the input for a hash function is always, in
* concept, an ordered byte array. This hashing API accepts an arbitrary
* sequence of byte and multibyte values (via {@link Hasher}), but this is
* merely a convenience; these are always translated into raw byte sequences
* under the covers.
*
* <li><b>hash code:</b> each hash function always yields hash codes of the same
* fixed bit length (given by {@link #bits}). For example, {@link Hashing#sha1}
* produces a 160-bit number, while {@link Hashing#murmur3_32()} yields only 32
* bits. Because a {@code long} value is clearly insufficient to hold all hash
* code values, this API represents a hash code as an instance of
* {@link HashCode}.
*
* <li><b>pure function:</b> the value produced must depend only on the input
* bytes, in the order they appear. Input data is never modified.
* {@link HashFunction} instances should always be stateless, and therefore
* thread-safe.
*
* <li><b>collision-averse:</b> while it can't be helped that a hash function
* will sometimes produce the same hash code for distinct inputs (a
* "collision"), every hash function strives to <i>some</i> degree to make this
* unlikely. (Without this condition, a function that always returns zero could
* be called a hash function. It is not.)
* </ul>
*
* <p>
* Summarizing the last two points: "equal yield equal <i>always</i>; unequal
* yield unequal <i>often</i>." This is the most important characteristic of all
* hash functions.
*
* <h3>Desirable properties</h3>
*
* <p>
* A high-quality hash function strives for some subset of the following
* virtues:
*
* <ul>
* <li><b>collision-resistant:</b> while the definition above requires making at
* least <i>some</i> token attempt, one measure of the quality of a hash
* function is <i>how well</i> it succeeds at this goal. Important note: it may
* be easy to achieve the theoretical minimum collision rate when using
* completely <i>random</i> sample input. The true test of a hash function is
* how it performs on representative real-world data, which tends to contain
* many hidden patterns and clumps. The goal of a good hash function is to stamp
* these patterns out as thoroughly as possible.
*
* <li><b>bit-dispersing:</b> masking out any <i>single bit</i> from a hash code
* should yield only the expected <i>twofold</i> increase to all collision
* rates. Informally, the "information" in the hash code should be as evenly
* "spread out" through the hash code's bits as possible. The result is that,
* for example, when choosing a bucket in a hash table of size 2^8, <i>any</i>
* eight bits could be consistently used.
*
* <li><b>cryptographic:</b> certain hash functions such as
* {@link Hashing#sha512} are designed to make it as infeasible as possible to
* reverse-engineer the input that produced a given hash code, or even to
* discover <i>any</i> two distinct inputs that yield the same result. These are
* called <i>cryptographic hash functions</i>. But, whenever it is learned that
* either of these feats has become computationally feasible, the function is
* deemed "broken" and should no longer be used for secure purposes. (This is
* the likely eventual fate of <i>all</i> cryptographic hashes.)
*
* <li><b>fast:</b> perhaps self-explanatory, but often the most important
* consideration. We have published <a href="#noWeHaventYet">microbenchmark
* results</a> for many common hash functions.
* </ul>
*
* <h3>Providing input to a hash function</h3>
*
* <p>
* The primary way to provide the data that your hash function should act on is
* via a {@link Hasher}. Obtain a new hasher from the hash function using
* {@link #newHasher}, "push" the relevant data into it using methods like
* {@link Hasher#putBytes(byte[])}, and finally ask for the {@code HashCode}
* when finished using {@link Hasher#hash}. (See an {@linkplain #newHasher
* example} of this.)
*
* <p>
* If all you want to hash is a single byte array, string or {@code long} value,
* there are convenient shortcut methods defined directly on
* {@link HashFunction} to make this easier.
*
* <p>
* Hasher accepts primitive data types, but can also accept any Object of type
* {@code
* T} provided that you implement a {@link Funnel Funnel<T>} to specify how to
* "feed" data from that object into the function. (See
* {@linkplain Hasher#putObject an example} of this.)
*
* <p>
* <b>Compatibility note:</b> Throughout this API, multibyte values are always
* interpreted in <i>little-endian</i> order. That is, hashing the byte array
* {@code {0x01, 0x02, 0x03, 0x04}} is equivalent to hashing the {@code int}
* value {@code
* 0x04030201}. If this isn't what you need, methods such as
* {@link Integer#reverseBytes} and {@link Ints#toByteArray} will help.
*
* <h3>Relationship to {@link Object#hashCode}</h3>
*
* <p>
* Java's baked-in concept of hash codes is constrained to 32 bits, and provides
* no separation between hash algorithms and the data they act on, so alternate
* hash algorithms can't be easily substituted. Also, implementations of
* {@code hashCode} tend to be poor-quality, in part because they end up
* depending on <i>other</i> existing poor-quality {@code hashCode}
* implementations, including those in many JDK classes.
*
* <p>
* {@code Object.hashCode} implementations tend to be very fast, but have weak
* collision prevention and <i>no</i> expectation of bit dispersion. This leaves
* them perfectly suitable for use in hash tables, because extra collisions
* cause only a slight performance hit, while poor bit dispersion is easily
* corrected using a secondary hash function (which all reasonable hash table
* implementations in Java use). For the many uses of hash functions beyond data
* structures, however, {@code Object.hashCode} almost always falls short --
* hence this library.
*
* @author Kevin Bourrillion
* @since 11.0
*/
@Beta
public interface HashFunction {
/**
* Begins a new hash code computation by returning an initialized, stateful
* {@code
* Hasher} instance that is ready to receive data. Example:
*
* <pre>
* {
* @code
*
* HashFunction hf = Hashing.md5();
* HashCode hc = hf.newHasher().putLong(id).putBoolean(isActive).hash();
* }
* </pre>
*/
Hasher newHasher();
/**
* Begins a new hash code computation as {@link #newHasher()}, but provides a
* hint of the expected size of the input (in bytes). This is only important for
* non-streaming hash functions (hash functions that need to buffer their whole
* input before processing any of it).
*/
Hasher newHasher(int expectedInputSize);
/**
* Shortcut for {@code newHasher().putInt(input).hash()}; returns the hash code
* for the given {@code int} value, interpreted in little-endian byte order. The
* implementation <i>might</i> perform better than its longhand equivalent, but
* should not perform worse.
*
* @since 12.0
*/
HashCode hashInt(int input);
/**
* Shortcut for {@code newHasher().putLong(input).hash()}; returns the hash code
* for the given {@code long} value, interpreted in little-endian byte order.
* The implementation <i>might</i> perform better than its longhand equivalent,
* but should not perform worse.
*/
HashCode hashLong(long input);
/**
* Shortcut for {@code newHasher().putBytes(input).hash()}. The implementation
* <i>might</i> perform better than its longhand equivalent, but should not
* perform worse.
*/
HashCode hashBytes(byte[] input);
/**
* Shortcut for {@code newHasher().putBytes(input, off, len).hash()}. The
* implementation <i>might</i> perform better than its longhand equivalent, but
* should not perform worse.
*
* @throws IndexOutOfBoundsException if {@code off < 0} or
* {@code off + len > bytes.length} or
* {@code len < 0}
*/
HashCode hashBytes(byte[] input, int off, int len);
/**
* Shortcut for {@code newHasher().putUnencodedChars(input).hash()}. The
* implementation <i>might</i> perform better than its longhand equivalent, but
* should not perform worse. Note that no character encoding is performed; the
* low byte and high byte of each {@code char} are hashed directly (in that
* order).
*
* @since 15.0 (since 11.0 as hashString(CharSequence)).
*/
HashCode hashUnencodedChars(CharSequence input);
/**
* Shortcut for {@code newHasher().putString(input, charset).hash()}. Characters
* are encoded using the given {@link Charset}. The implementation <i>might</i>
* perform better than its longhand equivalent, but should not perform worse.
*/
HashCode hashString(CharSequence input, Charset charset);
/**
* Shortcut for {@code newHasher().putObject(instance, funnel).hash()}. The
* implementation <i>might</i> perform better than its longhand equivalent, but
* should not perform worse.
*
* @since 14.0
*/
<T> HashCode hashObject(T instance, Funnel<? super T> funnel);
/**
* Returns the number of bits (a multiple of 32) that each hash code produced by
* this hash function has.
*/
int bits();
}